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USING THE 8340B/41B POWER CONTROL
AND MODULATION SYSTEMS.
The preceding explanations of the power control and modulation functions are sufficient for the
majority of applications; however, to extract the utmost performance from this instrument the follow¬
ing special information might be helpful.
INTERNAL LEVELING
The RF Output is controlled by the automatic level control (ALC) circuit, otherwise referred to as the
leveling loop. Figure 3-29 shows a simplified diagram of this system. The leveling loop is a feedback
control system, in which the output power is measured and compared to the desired level. If the two
are not equal the loop changes the output until they are equal.
The two inputs labeled "ALC INPUTS" convey the desired power level. One of these is a voltage
derived from the power value shown in the ENTRY DISPLAY. In the absence of modulation this
voltage is used to set the output power level. The amplitude modulation (AM) input causes the output
to increase or decrease relative to this level. (The pulse modulation input is essentially an ON/OFF
switch, not an ALC input.)
The RF power level from the level control circuits is referred to as the "ALC level," and is measured by
a crystal detector. The DC output from this detector is fed back to the level control circuits for
comparison with the ALC inputs. Since crystal detectors lose sensitivity at low power levels, the
detector provides an accurate power indication for ALC levels down to —10 dBm, and is acceptable
(±1dB) down to -20 dBm. The maximum amount of power available from the level control circuits
varies with RF frequency, from +1 dBm specified at 26.5 GHz (HP 8340B only) to typically +21 dBm at
4.5 GHz; therefore, the level control circuits can provide continuous control of ALC levels over a
maximum span of approximately —20 dBm to +20 dBm.
Coupled Mode
Since many applications require power levels less than —20 dBm, a step attentuator1 is provided that
has a range of 0 to -90 dB in 10 dB steps. Thus, power levels down to -110 dBm is achieved when
the attenuator and ALC work in conjunction. Because of the attenuator, the ALC will normally be used
over only a portion of its 40 dB range: Since accuracy suffers below — 10dBm and at some (HP
8340B) frequencies only +1 dBm is available, the ALC is normally set between -10 and 0 dBm. To get
power less than —100 dBm, the attenuator is left at —90dB, and the ALC used from —10 to —20 dBm;
however, the ALC accuracy and noise performance is degraded at this level, and is the reason that
some specifications apply only down to -100 dBm. At frequencies where power levels above 0 dBm
are desired, the attenuator is left a 0 dB and the ALC used from 0 to +20 dBm (or whatever power is
available at the RF frequency in use). The proper combination of ALC and attenuator is decided by the
internal microprocessor:
the user need only set the desired power in the ENTRY DISPLAY via the
[POWER] key.
i HP 8340B's equipped with Options 001 or 005 are not supplied with the step attenuator.
3-98
Operating Information
HP 8340B/41B
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